The present disclosure relates to ultrasonic imaging. More particularly, a system and method for improved contrast-agent enhanced-diagnostic evaluations are disclosed.
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. Ultrasonic images are typically formed through the use of phased or linear-array transducers which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. These ultrasonic transducers may be further assembled into a housing, which may contain control electronics, the combination of which forms an ultrasonic probe. Ultrasonic probes are used along with an ultrasonic transceiver to transmit and receive pressure waves through the various tissues of the body. The various ultrasonic responses are then processed by an ultrasonic-imaging system to display the various structures and tissues of the body.
Some ultrasound-imaging systems can create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the received ultrasonic echoes. In another common imaging modality, typically known as color-flow imaging, the flow of blood or movement of tissue is observed. Color-flow imaging modalities take advantage of the Doppler effect to color-encode image displays. In color-flow imaging, the frequency shift of backscattered ultrasound waves is used to measure the velocity of the backscatterers from tissues or blood. The frequency of sound waves reflecting from the inside of blood vessels, heart cavities, etc. is shifted in proportion to the velocity of the blood cells. The frequency of ultrasonic waves reflected from cells moving towards the transducer is positively shifted. Conversely, the frequency of ultrasonic reflections from cells moving away from the transducer is negatively shifted. The Doppler shift may be displayed using different colors to represent speed and direction of flow. To assist diagnosticians and operators, the color-flow image may be superimposed on the B-mode image.
Current color-flow imaging techniques have disadvantages in that it is difficult to obtain diagnostic quality images from patients that have a poor acoustic window (i.e., patients having a relatively large volume of tissue between their skin and their rib cage for heart related studies). In addition, it is often difficult to separate desired blood-velocity signals from artifacts that result from moving tissue. This problem is most severe when there is not a relatively large difference in velocity between the tissue and the blood contained therein.
Ultrasonic imaging can be particularly effective when used in conjunction with contrast agents. In contrast-agent imaging, gas filled micro-sphere contrast agents known as microbubbles are typically injected into a medium, normally the bloodstream. Due to their physical characteristics, contrast agents stand out in ultrasound examinations and therefore can be used as markers that identify the amount of blood flowing to or through the observed tissue. In particular, the contrast agents resonate in the presence of ultrasonic fields producing radial oscillations that can be easily detected and imaged. Normally, this response is imaged at the second harmonic of the transmit frequency, fo. By observing anatomical structures after introducing contrast agents, medical personnel can significantly enhance imaging capability for diagnosing the health of blood-filled tissues and blood-flow dynamics within a patient""s circulatory system. For example, contrast-agent imaging is especially effective in detecting myocardial boundaries, assessing micro-vascular blood flow, and detecting myocardial perfusion.
U.S. Pat. No. 5,410,516 to Uhlendorf et al. discloses contrast-agent imaging along with single-pulse excitation techniques such as harmonic imaging. Specifically, Uhlendorf teaches that by choosing a radio frequency (RF) filter to selectively observe any integer harmonic (2nd, 3rd, etc.), subharmonic (e.g., xc2xd harmonic) or ultraharmonic (e.g., {fraction (3/2)} harmonic) it is possible to improve the microbubble to tissue ratio. The second harmonic has proven most useful due to the large bubble response at this frequency as compared to higher-order integer harmonics, subharmonics or ultraharmonics. The second harmonic also is most practical due to bandwidth limitations on the transducer (i.e.,  less than 70% bandwidth, where percent bandwidth is defined as the difference of the high comer frequency xe2x88x926 dB point from the low comer frequency xe2x88x926 dB point, divided by the center frequency.) However, single-pulse excitation techniques together with harmonic imaging suffer from poor microbubble-to-tissue ratios as large tissue generated integer-harmonic signals mask the signals generated by the contrast agent.
As a result, of the discrimination problem associated with single-pulse excitation techniques, many multiple-pulse methodologies have been developed to suppress ultrasonic responses from anatomical tissues. These multiple-pulse excitation techniques result in diagnostic displays having an intensity that is responsive to the concentration of the contrast agent within the local insonified region.
Recently, it has been determined that tissue also produces harmonic responses which influence the images produced during contrast imaging. Several techniques have been developed which take advantage of the primarily linear response behavior of tissue to cancel or attenuate the linear-tissue signals. In several of these techniques, multiple-transmit lines are fired along the same line of sight into the body. The transmit waveform is modified (e.g., in terms of power, phase, or polarity) from line-to-line to produce a variation in the response received by the transducer. These data points are then processed to remove the influence of their linear components to yield data that primarily contains the non-linear response of the contrast agents.
Although the above-described techniques work well in removing the influence of stationary tissue, flash artifacts from moving tissue can degrade the resultant images. In particular, this movement causes decorrelation of the received echoes that is not compensated by typical processing techniques. This degradation can be substantial, particularly where the heart is being imaged due to its frequent and rapid motions. Attempts have been made to reduce the effects of such movement by applying two-zero filters to the responses of the receive signals associated with the various transmit lines. However, this technique assumes perfectly linear tissue movement and therefore is not completely effective in removing the moving tissue signals.
From the above, it can be appreciated that it would be desirable to have a method for contrast imaging in which the response of moving tissue is effectively suppressed so as to enhance the imaging sensitivity of the contrast agents. It will be further appreciated that it would be desirable to have a method for contrast imaging in which the response of moving tissue is effectively suppressed to permit quantitative assessment of flow velocities.
The present disclosure relates to apparatus and methods for imaging contrast agents within a patient""s body. More specifically, the disclosure relates to a system and method that improves contrast-agent enhanced-diagnostic evaluations by applying a color-flow processing algorithm to ultrasonic-response data generated and processed by a power-modulation technique selected for the ability to suppress tissue signals. The method generally comprises introducing one or more contrast agents into the body, transmitting power-modulated ultrasonic pulses into the body, receiving echoed signals from the body, processing the received data (in the received echo signals) to suppress tissue-response signals, processing the received contrast-agent signals with a color-flow algorithm, examining the color-flow processed contrast-agent signals over time to generate a contrast-agent velocity estimate, and generating a color-encoded display of contrast-agent velocities.
In one embodiment, the ultrasound signal comprises a plurality of signal lines that have been modulated to have different transmit characteristics. The step of processing the received data to suppress tissue-motion generated response signals comprises repetitively applying a weighted finite-impulse-response (FIR) filter to slow-time samples to substantially remove tissue signals and applying the received sequence to a one-zero clutter filter. This has the effect of removing signals from stationary contrast-agent bubbles, while passing signals generated by moving contrast-agent bubbles.
In an alternative embodiment, the velocity of the moving tissue is measured to generate a secondary velocity signal. This secondary velocity signal is then mathematically combined with the velocity estimate from the power-modulated flow signal, producing a corrected measurement of the blood-flow velocity relative to the surrounding tissue, rather than relative to the ultrasonic probe (e.g., the transducer).
Briefly described, in architecture, the system can be implemented with an excitation signal source, a transducer, an ultrasound-processing system having multiple-image processors including a color-flow processor, as well as, a clutter-filter, and an arbiter, and a display-processing system.
Other features and advantages of the system and method for contrast-agent enhanced color-flow imaging will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein as protected by the accompanying claims.